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Ok, so I dug out some old notes that served me well when I wanted to know what 100% charged is.
Not that I am going to charge to 100% all the time, but I needed a baseline to make my compensatory algo's for lower levels of charge.
Here we assume a typical CC/CV charge from a STABLE source of supply, not solar. Basically, the higher the voltage, the faster the absorb cutoffs. I'll do a simple single-cell vs a typical 12v (4s) batt to make it easier to reference:
3.60v (14.4v cv) end amps = C/20
3.55v (14.2v ) end amps = C/60
3.50v (14.0v ) end amps = C/100
3.45v (13.8v ) end amps = zero << conservative, but does NOT mean float!!!
Generally, going beyond the end taper current is overcharge. In fact, that's a misnomer - charge is over, BUT continuing further merely aggravates secondary reactions, which we don't want.
What you'll see here if you actually do this test with say your own gear for charge and discharge measurements, is that the higher voltage (3.6v) will get the most capacity in the shortest amount of time. BUT, the taper is so quick, you had better be on top of things, or you'll just be aggravating secondary reactions.
At the most conservative voltage, sure enough you can reach 100% charge, but that taper takes an excruciatingly long time to achieve. So much so, that in a solar situation, you'll never reach 100% before the sun goes down - depends of course on how much you have discharged from the battery first obviously.
That low 13.8v sure looks like a typical sla "float voltage" or output from a typical mobile radio power supply right? Well there's a big catch!
Sure it is "conservative", and give you plenty of time to stop charge before reaching 100%. BUT, if you charge like that, you'll be spending an awful lot of ** TIME ** going from say 98 to 100% - many hours! For all intents and purposes, that is "fully charged" enough so that you are aggravating secondary reaction stages, and you are spending waaay too much time trying to achieve that last little bit of capacity - it won't cost you today, but it will cost you down the road.
NOTE: So what does the secondary-reaction stage look like when you go beyond these taper-current stop points? A sudden rise in cell voltage, despite just sitting there at zero amps for awhile, which doesn't make sense from an electrical physics standpoint. Heh, that's right - because we are no longer dealing with charging per se, but we are now changing from battery chemistry into something quite a bit different. It's a weird concept, but I think crucial to understand.
How this secondary reaction is hidden! For many, using a BMS that puts a voltage cap set of bleeders on the cells hides the fact that you've gone beyond charging into secondary chemical reaction states by exceeding the taper current vs voltage values. In other words, the cell is ALREADY full and balanced. But then, as the voltage is rising, with no input current to accompany it coming from secondary reactions, and not normal charging, the bleeders activate and may pull the cells down to 100% charge and stop.
And we know that having a battery sitting around at 100% charge is not healthy over time. Again, we're talking about stable charge sources, not solar.
SOLAR: Ideally, a charge controller should be able to look at all those differences in the charts above, and if you actually desired a 100% charge, stop at any one of those taper currents as conditions vary. Yeah, the typical sla-based controller is not that smart to do this.
So like all things solar, reach a compromise. The simplest might be to simply stop once a voltage is reached, and not taper (or absorb) at all. Do a discharge test on your own, and see how much capacity you have. OR, perhaps stop a bit before these values to reach your desired objective (70/80/90 percent etc)
The values above are not hard facts. It is a starting point for you to test your own cells with a discharge capacity test if you want to fine tune it.
In the end, from a *time* exposure standpoint - if you have the ability to get your cells fully charge on a cyclic-standpoint, perhaps it is best to charge as fast as you can with a higher voltage, and get it over and done with. On the other hand, the lowest voltage cv setpoint may be ok for some who don't need full capacity - which can be reached given enough time, BUT watch that clock! Spending too much time per cycle to get to 100 % will cost you.
Now I see why some have recommended a middle-ground of using say 14.0 to 14.2v as their cv setpoint to make things not so "hairy edge" with high cv voltages, nor too much time getting a lot of charge in.
Not that I am going to charge to 100% all the time, but I needed a baseline to make my compensatory algo's for lower levels of charge.
Here we assume a typical CC/CV charge from a STABLE source of supply, not solar. Basically, the higher the voltage, the faster the absorb cutoffs. I'll do a simple single-cell vs a typical 12v (4s) batt to make it easier to reference:
3.60v (14.4v cv) end amps = C/20
3.55v (14.2v ) end amps = C/60
3.50v (14.0v ) end amps = C/100
3.45v (13.8v ) end amps = zero << conservative, but does NOT mean float!!!
Generally, going beyond the end taper current is overcharge. In fact, that's a misnomer - charge is over, BUT continuing further merely aggravates secondary reactions, which we don't want.
What you'll see here if you actually do this test with say your own gear for charge and discharge measurements, is that the higher voltage (3.6v) will get the most capacity in the shortest amount of time. BUT, the taper is so quick, you had better be on top of things, or you'll just be aggravating secondary reactions.
At the most conservative voltage, sure enough you can reach 100% charge, but that taper takes an excruciatingly long time to achieve. So much so, that in a solar situation, you'll never reach 100% before the sun goes down - depends of course on how much you have discharged from the battery first obviously.
That low 13.8v sure looks like a typical sla "float voltage" or output from a typical mobile radio power supply right? Well there's a big catch!
Sure it is "conservative", and give you plenty of time to stop charge before reaching 100%. BUT, if you charge like that, you'll be spending an awful lot of ** TIME ** going from say 98 to 100% - many hours! For all intents and purposes, that is "fully charged" enough so that you are aggravating secondary reaction stages, and you are spending waaay too much time trying to achieve that last little bit of capacity - it won't cost you today, but it will cost you down the road.
NOTE: So what does the secondary-reaction stage look like when you go beyond these taper-current stop points? A sudden rise in cell voltage, despite just sitting there at zero amps for awhile, which doesn't make sense from an electrical physics standpoint. Heh, that's right - because we are no longer dealing with charging per se, but we are now changing from battery chemistry into something quite a bit different. It's a weird concept, but I think crucial to understand.
How this secondary reaction is hidden! For many, using a BMS that puts a voltage cap set of bleeders on the cells hides the fact that you've gone beyond charging into secondary chemical reaction states by exceeding the taper current vs voltage values. In other words, the cell is ALREADY full and balanced. But then, as the voltage is rising, with no input current to accompany it coming from secondary reactions, and not normal charging, the bleeders activate and may pull the cells down to 100% charge and stop.
And we know that having a battery sitting around at 100% charge is not healthy over time. Again, we're talking about stable charge sources, not solar.
SOLAR: Ideally, a charge controller should be able to look at all those differences in the charts above, and if you actually desired a 100% charge, stop at any one of those taper currents as conditions vary. Yeah, the typical sla-based controller is not that smart to do this.
So like all things solar, reach a compromise. The simplest might be to simply stop once a voltage is reached, and not taper (or absorb) at all. Do a discharge test on your own, and see how much capacity you have. OR, perhaps stop a bit before these values to reach your desired objective (70/80/90 percent etc)
The values above are not hard facts. It is a starting point for you to test your own cells with a discharge capacity test if you want to fine tune it.
In the end, from a *time* exposure standpoint - if you have the ability to get your cells fully charge on a cyclic-standpoint, perhaps it is best to charge as fast as you can with a higher voltage, and get it over and done with. On the other hand, the lowest voltage cv setpoint may be ok for some who don't need full capacity - which can be reached given enough time, BUT watch that clock! Spending too much time per cycle to get to 100 % will cost you.
Now I see why some have recommended a middle-ground of using say 14.0 to 14.2v as their cv setpoint to make things not so "hairy edge" with high cv voltages, nor too much time getting a lot of charge in.
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